We have just completed analysis of the amino acid sequence of wild-type Histidinol dehydrogenase from Salmonella typhimurium. Work is in progress on a number of mutants which result in altered properties of the enzyme. We shall investigate the nature of the biochemical defects, and establish the underlying change in amino acid sequence. Chemical modifications are being carried out to identify residues involved in catalytic activity, substrate and coenzyme binding. Approaches to be used include photooxidation, covalent modification, etc. Intragenic complementation will be studied using mutants which show no activity in the homozygous condition, but which allow function when combined in heterozygotes. We shall attempt to achieve complementation between mutant enzymes in vitro by subunit reassociation.